Rising sea level will kill coral reefs, because corals
can't
grow fast enough to stay near the surface.
Too much CO2 in the water will inevitably kill corals. The
temperature can also kill them. So it seems that coral has little
chance of surviving to the end of this century.
A billion people depend on marine resources for their food. In a few
decades it could be a billion and a half. Thus, the loss of most of
those resources could easily kill a billion people.
They won't all die directly from hunger. Many of them will die in
wars over control of the dwindling marine resources. These wars will
be futile, in that victory will at most delay the death of the
victors.
If they start sooner, and fight fossil fuel extraction and use, they
might avert much of the disaster. However, if they don't limit their
populations, then barring some unforeseeable event they will reach the
starvation point sooner or later.